A NEW  TYPE  OF  LETHAL  HEREDITARY  FACTOR 
IN  DROSOPHILA 


BY 

DAVID  HIRAM  THOMPSON 
B.  S.  Purdue  University 
1919 


THESIS 

Submitted  in  Partial  Fulfillment  of  the  Requirements  for  the 

Degree  of 

MASTER  OF  SCIENCE 
IN  ZOOLOGY 

IN 

THE  GRADUATE  SCHOOL 

OF  THE 

UNIVERSITY  OF  ILLINOIS 


1921 


\°tl\ 

-T'11 


UNIVERSITY  OF  ILLINOIS 


THE  GRADUATE  SCHOOL 


May , 20 


§ 


I HEREBY  RECOMMEND  THAT  THE  THESIS  PREPARED  UNDER  MY 
SUPERVISION  ry  David  Hiram  Thompson 


ENTITLED AJEQ5W  TYPE  0?  LETHAL  HWRTTTARY  PAOTOR 


IK  DROSOPHILA 


BE  ACCEPTED  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR 

THE  DEGREE  OF Master  of  Science  in  ?nAir,gy 

2, 


in  Charge  of  Thesis 


eacrof  Department 


Recommendation  concurred  in* 


Committee 


on 


Final  Examination* 


Required  for  doctor  s degree  but  not  for  master’ 


A HEW  TYPE  OP  LETHAL  HEREDITARY  FACTOR  IE  DROSOPHILA 


Contents 

Page 

I Introduction X 

II  Method  of  culture  and  maintenance  of  stock,, 2 

III  Origin  of  erect 

IV  Sex  ratios  and  frequency  of  erect .......... .4 

V Location  of  erect  factor .....9 

VI  Discussion .,,,.11 

VII  Summary, X8 


Digitized  by  the  Internet  Archive 
in  2016 


https://archive.org/details/newtypeoflethalhOOthom 


1 


I INTRODUCTION 

The  question  of  the  viability  of  females  homozygous  for 
sex -linked  lethals  was  raised  by  Morgan  and  Bridges  (1916). 

At  that  time  they  were  unable  to  state  definitely  that  a 
female  with  such  a germinal  constitution  would  not  live  but 
they  made  that  assumption.  A sex -linked  lethal  has  been  found 
in  which  the  lethal  bearing  males  live  to  transmit  the  lethal 
factor.  This  has  made  it  possible  to  test  the  case  and  their 
assumption  was  found  to  be  correct. 

About  two  years  ago  there  appeared  in  a cross  between  a 
wild  female  and  a white  ultra-bar  several  males  which  bore 
a new  sex-linked  character.  In  this  character  the  wings  are 
held  erect  over  the  back,  the  second  pair  of  legs  is  function- 
less, and  on  the  whole,  these  males  resemble  flies  which  have 
been  o vex-* -etherized . When  females  heterozygous  for  this  char- 
acter are  mated  with  erect  males  there  appear  in  the  next 
generation  about  equal  numbers  of  erect  sons,  normal  sons  and 
normal -appearing  females.  But  there  are  no  erect  females. 
Moreover  none  of  these  females  have  been  found  to  be  homozygous 
for  the  character  and  the  sex  ratio  of  two  males  to  one  female 
is  always  obtained  in  such  a cross. 

The  sex  ratio  is  of  interest  because  it  is  the  reverse 
of  the  sex  ratio  found  in  other  sex-linked  lethals.  It  also 
suggests  the  lethal  (lgd)  described  by  Miss  Stark,  in  which 
the  lethal  bearing  males  occasionally  emerged  and  lived  for 
a short  time,  although  they  were  unable  to  use  their  legs  for 
walking.  The  present  lethal  is  similiar  in  that  the  lethal 


"bearing  males  emerge  and  the  legs  are  often  useless.  However 
in  this  case  the  lethal  hearing  males  live  to  propagate  and 
if  mated  with  females  heterozygous  for  erect  the  lethal  effect 

is  obtained  in  one  half  of  the  females. 

£ 

II  METHOD  OF  CULTURE  AND  MAINTENANCE  OF  STOCK 

The  flies  have  been  reared  by  the  ordinary  method  and 
kept  in  a room  with  a controlled  temperature  of  24  C.  When  it 
is  desired  to  examine  the  flies  they  are  very  lightly  etherized 
and  the  erect  flies  quickly  separated  from  the  non-erects, 
after  which  they  may  be  more  heavily  etherized.  This  precaution 
is  taken  in  order  to  avoid  confusion  of  erect-winged  flies  with 
over-etherized  flies.  This  method  of  separation  and  classific- 
ation of  erect-winged  flies  is  effective  since  no  fly  has  been 
found  with  normal  wings  when  under  the  influence  of  ether  which 
later  showed  erect  wings  and  conversely  no  fly  classified  as 
having  erect  wings  has  later  shown  the  normal  wing  posture.  In 
matings  involving  erect  the  offspring  should  be  shaken  out  and 
classified  each  day  if  close  approximations  to  expected  ratios 
are  desired,  since  any  flies  which  may  die  ,due  to  crowding, 
cannot  be  classified.  Classifications  are  made  under  the  low 
power  of  a binocular  microscope  although  they  can  be  made  equal- 
ly well  with  the  naked  eye. 

A stock  pure  for  erect  is  not  possible  due  to  the  total 
absence  of  females  homozygous  for  erect.  A stock  of  erect  has 
been  maintained  by  mating  females  heterozygous  for  erect  with 
erect  males  in  each  generation.  In  such  a mating  one  always 
gets  one  half  erect  sons  and  one  half  normal  sons  and  a sex 
ratio  of  two  males  to  one  female.  The  stock  has  been  examined 


« 


in  each  generation  for  any  marked  deviation  in  the  one  to  two  ^ 
sex  ratio  and  in  the  percentage  of  erect  males.  A close  watch 
has  been  made  for  homozygous  females. 

Ill  ORIGIN  OP  PRPCT  ! 

On  September  P,  1919,  in  a.  cross  between  a wild  female  and  J 
a white  ultra  bar  male  there  were  noticed  several  males  crawl- 
ing about  the  bottle  with  wings  erect  over  the  thorax  as  in 
over-etherized  flies.  This  day’s  count  of  offspring  consisted 
of  35  normal -winged  females,  10  normal -winged  males  and  14 
erect  males.  This  female  evidently  was  heterozygous  for  erect  and 
since  she  was  a virgin  picked  from  a wild  stock  the  character 
derived  from  her  cannot  be  said  to  have  arisen  by  hybridization 
of  two  stocks  but  from  a pure  wild  stock.  It  may  be  of  interest 
to  note  in  connection  with  Miss  Stark’s  work  (1915)  on  lethals 
from  freshly  caught  and  inbred  stocks  that  the  wild  stock  in 
which  this  erect  lethal  arose  had  been  in  captivity  for  at  least 
three  or  four  years. 

The  erect  males  which  must  have  been  present  in  earlier 
counts  of  the  original  cross  were  overlooked  probably  because 
they  were  supposed  to  be  over-etherized  flies.  The  resemblence 
of  these  erect  males  to  over-etherized  flies  is  striking. 

In  the  character  ’’erect"  it  is  the  raesothorax  which  seems 
to  be  affected  most  because  the  wings  are  held  erect  over  the 
back  and  the  second  pair  of  legs  is  usually  doubled  up  beneath 
the  body  or  else  dragged  along  behind  (figure  1).  The  wings 
stand  at  right  angles  with  the  main  axis  of  the  body  and  they 
usually  are  about  parallel  but  occasionally  touch  or  form  an  j 

acute  angle.  The  wings  come  into  position  as  they  expand  and 


4 


remain  so  throughout  the  life  of  the  fly,  thus  making  flight 
impossible.  No  males  have  been  found  that  transmit  the  factor 
for  erect  without  having  erect  wings,  and  conversely  no  erect 
males  have  been  found  which  do  not  transmit  the  characteristic. 

The  second  pair  of  legs  usually  are  useless, for  walking, as  they 

A 

are  either  crossed  and  doubled  up  beneath  the  body  or  else  they 
drag  as  the  animal  walks.  Such  flies  have  much  difficulty  in 
walking  and  occasionally  become  entangled  in  the  food  and  die. 
This  fact  probably  accounts  partially  for  th<^erects  appearing 
in  somewhat  fewer  numbers  than  expected.  Dead  flies  cannot  be 
classified  as  to  the  erect  character  and  are  discarded.  All 
figures  given  in  this  paper  refer  to  counts  of  living  flies. 
Flies  with  these  feeble  legs  often  partially  recover  the  use  of 
them  for  walking.  In  general  flies  cannot  be  classified  by  these 
feeble  legs  since  they  are  rather  irregular  in  appearance.  They 
are  part  of  the  manif estation  of  the  fa.ctor  for  erect,  because 
erect  flies  with  normal -appearing  legs  were  selected  for  several 
generations,  but  the  majority  of  the  erect-winged  males  always 
had  feeble  legs.  With  moderate  care  in  etherization  erect  is  a 
readily  recognized  sex-linked  character  which  is  always  definite 

in'jits  behavMfr. 

A 

IV  SEX  RATIOS  AND  FREQUENCY  OF  ERECT 
When  females  heterozygous  for  erect  are  out-crossed  to  non- 
erect  males  the  usual  1:1  sex  ratio  is  obtained  and  there  is 
an  approximation  to  equality  of  erect  and  normal  males.  The 
totals  of  such  crosses  are  given  in  table  1.  The  scarcity  of 
erect  males  here  is  partially  accounted  for  by  the  number  which 
died  before  they  were  counted,  as  the  offspring  were  not  removed 
daily  in  every  case. 


« 


TABLE  1 Heterozygous  female  X non-erect  male. 


Normal  $9 

Erect  dW 

Normal  eft? 

Total 

1252 

573 

749 

Percentage 

48,5 

22.3 

29.1 

Sex  Ratio 

1 : 

1.06 

6 


If  females  heterozygous  for  erect  he  mated  to  erect  males 
the  characteristic  sex  ratio  of  1:2  is  obtained  and  there  are 
again  nearly  equal  numbers  of  erect  and  normal  males.  The  totals 
of  these  matings  are  given  in  table  2.  It  can  be  seen  there  that 
tne  results  indicated  by  the  totals  are  not  the  averages  of  wide- 
ly varying . ratios , for  a close  approximation  to  the  expected 
ratio  has  been  obtained  in  almost  every  mating. 


. pi  v/  ji»<?  .8 


* 

- 


TABUS 

2 Heterozygous 

female  X 

erect  male. 

Culture 

number  Erect  9$ 

1'Iormal 

o$>  Erect 

oc?  Normal 

76,1 

0 

102 

101 

116 

84.1 

0 

30 

21 

30 

88.1 

0 

111 

104 

131 

86.1 

0 

52 

65 

64 

90.1 

0 

80 

110 

114 

92.1 

0 

99 

110 

10  5 

94.1 

0 

12 

15 

9 

96.1 

0 

4 

3 

3 

98.1 

0 

60 

54 

64 

100.1 

0 

52 

35 

48 

172.0 

0 

60 

55 

82 

172.1 

0 

90 

86 

87 

172.2 

0 

66 

94 

93 

172.3 

0 

91 

85 

120 

172.4 

0 

46 

45 

36 

172.5 

0 

20 

8 

12 

172.6 

0 

55 

65 

100 

172.7 

0 

38 

37 

51 

172.8 

0 

53 

54 

61 

172.9 

0 

51 

20 

23 

Total 

0 

1166 

1167 

1349 

Percentage 

0 

31.7 

31.7 

36.6 

Sex  Ratio 

1 

: 2.16 

Unrelated  non-erect  females  mated  to  erect  males  gave  in 
F-]_  equal  numbers  of  normal -appearing  females  and  males.  Many  of 
these  F1  females  were  then  tested  by  unrelated  non-erect  males 
and  all  gave  approximately  equal  numbers  of  erect  and  normal 
sons.  This  test  brings  out  its  characteristic  sex -linked  behav- 
/3\i/r. 

Several  thousand  flies  have  been  produced  in  crosses  in- 
volving erect,  and  a constant  watch  has  been  made  for  homozyg- 
ous erect  females  which  would  indicate  crossing-over  between  a 
lethal  factor  and  the  erect  factor,  but  no  such  females  have 
been  observed.  Since  the  lethal  effect  and  the  character  erect 
cannot  be  separated,  it  may  be  concluded  that  they  are  due  to 
the  same  factor. 

In  the  second  generation  from  a cross  between  white  bar 
females  and  erect  males  there  appeared  three  females  which  car- 
ried their  wings  erect.  These  upon  being  tested  proved  to  be 
heterozygous  for  erect  but  they  transmitted  this  character  of 
erect  wings  to  part  of  their  daughters.  Considerable  work  has 
been  done  in  testing  these  erect-winged  females  and  it  seems 
to  indicate  a dominant  factor  which  makes  erect  dominant  and 
is  lethal  in  the  homozygous  condition  if  one  erect  factor  is 
present.  This  new  factor  seems  to  be  located  in  the  autosomes, 
and.  unlike  the  erect  factor  it  is  quite  irregular  in  its  mani- 
festation since  certain  environmental  conditions  seem  to  be 
necessary.  This  new  dominating  factor  is  similiar  to  erect  in 
that  it  cannot  be  fixed,  at  least  when  the  erect  factor  is 
present. 

Homozygous  erect  females  probably  die  in  the  egg  or  early 
larval  stages  since  no  approximation  of  2$%  of  pupae  containing 


. 


dead  fliee  has  been  found  in  crosses  where  they  should  occur, 
and  no  dead  larvae  noticed  in  good  cultures. 

V LO CAT I OH  OH  ERECT  FACTOR 

In  order  to  prove  that  this  erect-winged  lethal  is  a sex- 

linked  character  its  linkage  relations  with  har  and  white  have 
"been  obtained.  White,  "bar,  normal  -winged  females  were  crossed 
to  red, full -eyed, erect  males  and  the  TV>  generation  produced. 

The  double-cross-over  class  was  regarded  as  a cross-over  in 
each  region.  The  males  of  this  cross  show  the  linkage  relations 
of  erect  with  "bar  and  white  readily  and  are  summarized  in  tables 
3 and  4.  There  was  found  to  be  36.9^  of  crossing -over  between 
white  and  erect  and  19.3^  between  bar  and  erect.  These  numbers 
added  give  close  approximation  of  56.2^  to  the  established 
5 6%  of  crossing-over  between  white  and  bar.  It  seems  evident 
that  the  factor  for  erect  lies  between  white  and  bar  somewhere 
near  the  locus  38* 


- 

< t • 

• 

» 1 

• 

. 

f 

TABLE  3 Bar-erect  crossing-over 


Cross-over  cftr  Non -cross -ever  cfd” 
Total  227  952 

Percentage  19*3 

TABLE  4 White-erect  crossing -over 

Cross-over  <fc?  Non-crcss-over  eft? 
Total  238  407 

Percentage 


36.9 


1 


* 


- 

- 


VI  DISCUSSION 


11 


An  hereditary  lethal  may  he  defined  as  any  heritable  qual- 
ity which  prevents  the  individual  from  reaching  the  adult  stage 
and  reproducing.  Lethal  effects  may  also  he  thought  of  as  includ- 
ing those  qualities  which  lower  the  viability  of  any  race  of 
animals  and  may  he  termed  racial  lethal s. 

The  first  case  of  an  hereditary  lethal  factor,  which  is 
quite  typical,  was  demonstrated  by  Cuenot  in  mice.  In  mice  there 
is  a yellow  coat  color  which  cannot  he  fixed,  that  is,  a line 
pure  for  yellow  cannot  he  obtained.  For  instance,  if  a pure  blaek 
he  crossed  to  a yellow,  one  half  of  the  offspring  are  yellow  and 
one  half  are  non-yellcw  and  it  is  not  evident  which  is  the  domin- 
ant  character.  But  by  further  breeding  experiments  it  is  shown 
that  yellow  is  dominant  because,  if  the  non-yellow  offspring  be 
mated  together,  none  but  non-yellows  are  obtained,  showing  that 
they  are  pure  for  the  absence  of  yellow.  If  the  yellow  offspring 
be  mated  to  the  non-yellow  equal  numbers  of  the  yellow  and  the 
non-yellow  are  again  obtained,  which  shows  that  yellow  is  domin- 
ant, since  it  could  have  been  present  in  the  offspring  only  in 
the  single  dose. 

Since  yellow  is  dominant  one  would  expect  that  if  he  mated 
yellow  offspring  together  he  would  get  yellows  and  non-  yellows 
in  the  ratio  of  3:1  and  of  the  composition  1YY  : 2Yy  : lyy,  but 
on  the  contrary  one  gets  a 2:1  ratio  as  was  established  by  Little 
(191?)  in  large  numbers  of  mice.  Furthermore,  one  does  not  get 
pure  yellow  mice  since  large  numbers  have  been  tested  and  all 
have  been  found  to  be  heterozygous  for  yellow.  It  was  noticed 
that  when  the  yellow  by  yellow  cross  was  made  the  litters  were 


< 


c 


t 


< 


t 


( 


smaller  than  normal,  being  about  three-fourths  as  large*  It  was 
then  concluded  that  it  was  the  pure  yellow  class  of  mice  which 
was  for  some  reason  missing  and  that  whenever  two  gametes  united, 
each  of  which  carried  the  yellow  factor,  the  resulting  zygote 
did  not  come  to  maturity. 

Embryo logical  work  has  recently  been  done  on  the  yellow  mice 
and  it  has  been  shown  that  when  two  yellow  gametes  unite  they 
form  a living  embryo  which  lives  an  apparently  normal  existence 


up  to  the  bias tula  stage,  when  the  embryos  become  implanted  in 


the  uterine  wall.  At  this  point  the  two  yellow  factors  manifest 
themselves,  the  embryo  does  not  become  attached  but  dies,  prob- 
ably from  lack  of  nourishment.  It  has  also  been  shown  that  the 
surviving  yellow  mice  are  mentally  deficient.  This  is  indicated 
by  their  behavoir  in  mazes  where  their  standard  of  performance 
is  much  lower  than  that  of  normal  mice. 

This  yellow  mouse  case  is  a simple,  clean  cut,  and  rather 
completely  understood  example  of  how  lethals  act. 


The  great  majority  of  known  hereditary  lethal  factors  are 
found  in  the  fruit  fly,  Drosophila,  and  the  largest  number  of 
these  are  sex-linked,  that  is,  the  factors  are  borne  by  the  sex- 
chromosomes.  This  is  because  all  known  lethal  factors  are  recess- 
ives,  and  sex-linked  recessives  are  very  easily  found  since  one 
half  the  sons  of  a female  heterozygous  for  any  one  of  them  are 
always  pure  for  it.  This  follows  since  there  are  two  sex-chromo- 
somes in  the  female  and  one  in  the  male  which  he  always  gets  from 
hi 8 mother. 

All  lethal  factors  must  be  recessive  in  their  lethal  effect 
in  order  to  be  inherited  because  it  is  obvious  that  if  they  were 


< 


13  I 


dominant  there  would  be  no  way  of  transmitting  them. 

The  number  of  dominant  lethals  arising  by  mutation  is  un- 
known but  there  are  two  cases  known  which  are  interesting  in  this 
connection.  Muller  (1921)  has  described  a sex-linked  lethal  which 
is  recessive  under  ordinary  conditions  of  the  germplasm  but  is  a 
dominant  lethal  when  a certain  other  allelomorph  for  this  lethal 
factor  is  present.  The  other  case  is  mentioned  in  this  paper  in 
which  the  erect  factor  seems  to  behave  as  a dominant  lethal  when 
two  doses  of  the  dominant  accessory  factor  are  present. 

More  is  known  of  recessive  than  of  dominant  lethals.  There 
ifl  a variety  of  these  which  differ  in  the  time  of  producing  death, 
as  well  as  the  degree  of  viability.  These  variations  seem  to  be 
well  illustrated  by  the  following  series  which  begins  with  the 
most  deadly  of  these  lethals  and  grades  down  to  their  milder 
forms . 

1.  The  first  lethal  to  be  considered  is  a sex-linked  recess- 
ive factor  reported  by  Miss  Stark  (1918).  This  causes  the  devel- 
opment of  tumors  in  the  male  larvae  and  kills  them  before  they 
pupate.  Since  this  factor  is  carried  by  the  sex-chromosomes  it 
follows  the  rules  for  sex-linked  inheritance.  It  can  only  be 
transmitted  by  the  females,  and  the  stock  is  maintained  by  mating 
heterozygous  females  to  normal  males  in  every  generation.  The 
sons  of  a heterozygous  female  should  be  equal  in  number  to  the 
daughters,  but  since  one  half  of  the  sons  die  as  larvae,  one  gets 
sons  and  daughters  in  the  ratio  of  two  daughters  to  one  son, 
which  is  the  usual  ratio  in  sex-linked  lethals. 

When  one  mates  females  heterozygous  for  the  lethal  to  normal 
males,  all  the  sons  that  come  to  the  mature  stage  are  normal  and 
cannot  transmit  the  defect.  The  daughters  are  of  two  classes. 


. 

« 

. 

. 


. 

. 

* 

. 


‘ 

- 


, 

, 


. 

, 


one  half  "being  heterozygous  for  the  lethal  factor  and  the  other 
half  "being  pure  for  its  absence.  As  a result  the  former  females 
will  always  transmit  the  defect  to  half  their  sons,  while  the 
latter  can  never  transmit  it*  The  principal  characteristic  of 
this  lethal  is  that  it  kills  one  half  of  the  males  in  the  larval 
stage  "by  the  development  of  tumors. 

2,  The  second  case  in  this  series  was  also  described  by 
Miss  Stark  (1915).  This  is  similiar  to  the  first  in  that  one 
half  of  the  sons  of  the  heterozygous  female  were  killed.  However 
no  evidence  of  a tumor  was  found.  The  males  receiving  the  lethal 
factor  lived  through  the  larval  stage  and  the  early  pupal  stages, 
but  died  either  before  they  emerged  or  shortly  afterwards.  Those 
that  emerged  could  not  walk,  but  fell  over  and  lay  on  their  sides, 
since  the  legs  did  not  seem  to  move  coordinately  and  were  not 
strong  enough  to  support  the  body.  These  flies  died  in  a day  or 
two  and  could  not  be  used  in  breeding  experiments.  The  points  of 
interest  in  this  lethal  were  that  one  half  of  the  males  were  af- 
fected as  in  the  previous  case,  but  that  the  effect  was  delayed 
to  the  late  pupal  or  young  adult  stages.  It  is  also  notable  in 
connection  with  the  following  that  the  lethal  bearing  males  which 

did  emerge  were  very  defective. 

3.  The  next  case  in  this  series  is  the  erect  lethal  describ- 
ed in  this  paper.  The  lethal  factor  is  transmitted  to  one  half 
the  sons  as  in  the  two  former  cases,  but  does  not  have  its  com- 
plete effect  in  them  since  they  go  through  the  whole  life  cycle 
and  emerge  as  adults.  However,  one  can  see  the  partial  effect  of 
the  lethal  because  the  wings  are  carried  erect  and  resemble  the 
the  condition  of  rigor  mortis.  Also  these  flies  have  difficulty 


in  walking  since  the  second  pair  of  legs  is  almost  always  para- 
lyzed and  is  dragged  along  as  the  animal  walks.  However  these 
lethal  hearing  males  live  to  breed  which  shows  an  advance  over  the 
last  case.  These  sons  which  bear  the  lethal,  when  mated  to  females 
heterozygous  for  it  should  give  the  following  types  of  offspring 
in  equal  numbers:  daughters  pure  for  the  lethal,  daughters  hetero- 
zygous for  the  lethal,  erect  sons  and  normal  sons,  One  does  not 
get  all  these  classes  of  flies  because  there  are  no  females  prod*» 
uced  which  are  pure  for  the  factor  and  therefore  two  males  sur- 
vive for  every  female.  This  shows  that  the  females  which  get 
two  doses  die  early  in  the  life  cycle.  This  stock  is  kept  going 
by  mating  heterozygous  females  to  erect  males. 

The  outstanding  points  are  that  the  factor  does  not  have  its 
complete  effect  in  the  lethal  bearing  males,  but  such  an  effect 
is  obtained  in  the  females  homozygous  for  it.  Here  the  ordinary 
lethal  sex  ratio  is  reversed  being  two  males  to  one  female  instead 
of  one  male  to  two  females. 

4.  For  the  next  case  in  this  series  the  character  fused  may 
be  taken  and  regarded  as  a mild  lethal  which  although  not  lethal 
to  the  individual  is  lethal  to  the  race  on  account  6f  the  ster- 
ility  of  all  homozygous  females.  Here  the  lethal  bearing  males 
live  as  in  the  erect  case  and  show  some  lethal  effects,  in  that 
the  ocelli  are  missing  and  the  wings  are  somewhat  abnormal.  These 
males  when  mated  to  heterozygous  females  give  all  the  expected 
classes  of  flies,  and  in  this  respect  are  different  from  the  prev- 
ious case. 

5«  The  factor  for  rudimentary  wing  behaves  in  the  same  way 
as  fused  except  that  the  homozygous  females  occasionally  lay  a 
few  eggs. 

I 


„ 

; 

< 

. 

« 


16 


6.  For  the  last  case  in  the  series  any  of  the  ordinary 
sex-linked  factors  may  he  mentioned  such  as  har  eye,  yellow 
body  etc.  In  these  instances  there  is  no  lethal  effect  causing 
certain  whole  classes  of  individuals  to  die  nor  is  there  com- 
plete sterility  as  in  other  cases  hut  these  stocks  have  a much 
lower  viability  than  the  normal  wild  type  of  fly. 

This  series  shows  the  different  points  in  the  life  cycle 
as  well  as  the  manner  in  which  the  lethals  may  manifest  them- 
selves. 

In  the  evening  primrose,  Oenothera  Lamarkiana,  there  are 
several  phenomena  which  have  puzzled  those  who  attempt  to  ex- 
plain heredity  on  the  determinate  basis.  Among  these  peculiar- 
ities are  the  constant  hybrids  which  apparently  are  races  pure 
for  certain  characters  although  they  are  really  heterozygous 
in  composition,  /mother  interesting  fact  is  that  when  two  appar- 
ently pure  races  are  crossed  there  appear  two  and  sometimes 
four  classes  of  F^  offspring.  It  has  been  shown  by  Muller  (1917) 
that  these  cases  can  be  paralleled  in  Drosophila  by  the  proper 
use  of  lethals. 

An  example  of  constant  hybrids  in  Drosophila  is  the  dom- 
inant character  ’’beaded  wings”  which  under  selection  became  a 
constant  hybrid.  The  principal  factor  is  a dominant  which  is 
lethal  in  the  double  dose  just  like  the  yellow  mouse  case. 

During  selection  a lethal  factor  appeared  which  was  closely 
linked  to  the  normal  allelomorph  for  beaded.  Thus  the  beaded 
stock  consisted  of  flies  heterozygous  for  both  beaded  and  the 
lethal.  If  B represents  the  beaded  factor  and  1 the  lethal 
factor  and  N their  normal  allelomorphs,  there  will  be  two  types 
of  eggs  and  two  types  of  sperm  with  regard  to  these  factors 


. 

• 

< 

( 

. 

. 

' 

. L 


• 

9 

. 

, 

* 

< 

( 

ITT  ♦ Neglecting  crossing -over 


namely  NB  and 

there  are  three  possible  combinations  of  these  two  types  of 
eggs  and  sperm  namely,  NNBB,  HEN,  and  1NBN.  The  first  class 
dies  as  a result  of  the  lethal  action  of  two  B*s  and  the  sec- 
ond as  a result  of  the  lethal  action  of  the  two  l's  while  the 
third  class  lives*  This  class  which  lives  is  of  the  same  comp- 
osition as  that  of  the  parents  and  it  repeats  as  a pure  line 
except  for  occasional  flies  of  other  types  produced  by  cross- 
ing-over between  the  lethal  and  beaded  factors*  If  this  appar- 
ently pure  beaded  stock  be  outcrossed  to  some  other  stock  such 
as  the  wild  fly  there  will  be  two  classes  of  $1  hybrids  namely 
one  half  beaded  flies  without  the  lethal  factor  and  one  half 
normal  flies  heterozygous  for  the  lethal.  By  crossing  two  diff- 
erent constant  hybrid  races  there  will  be  four  classes  of 
hybrids. 

By  making  use  of  the  idea  of  balanced  lethals  in  connect- 
ion with  the  erect  stock  one  should  be  able  to  produce  a stock 
in  which  all  the  females  would  be  normal  in  appearance  and 
the  males  all  erect.  This  should  be  done  by  introducing  an  ord- 
inary sex-linked  lethal  into  the  erect  stock  producing  females 
of  the  composition  1 • Here  the  ordinary  lethal  is 

le 

represented  by  1 and  the  erect  lethal  by  le.  These  females 
when  mated  to  erect  males  would  give  erect  sons  only,  the  nor- 
mal sons  being  killed  by  the  introduced  lethal.  The  daughters 
of  this  cross  would  be  of  the  same  composition  as  their  mother 
and  equal  in  number  to  the  erect  sons.  The  offspring  that  reach 
the  adult  stage  are  of  the  same  composition  as  the  parents  and 
the  process  would  repeat  itself  much  as  a pure  line.  Crossing- 


18 


over  would  give  normal  males  and  females  not  bearing  the  introd- 
uced lethal,  the  number  of  which  would  depend  on  the  closeness 
of  the  lethal  factor  to  the  erect  factor  or  the  presence  of  oth- 
er factors  which  may  affect  crossing -over.  This  balanced  lethal 
should  be  of  use  in  preserving  stocks  of  other  sex-linked  lethals 
since  it  would  give  a higher  percentage  of  flies  bearing  the  leth- 
al factor  than  other  methods. 

VII  SUMMARY 

1.  A female  from  an  inbred  wild  stock  has  been  found  to  be 
heterozygous  for  a new  sex-linked  receesive  character  in  which 
the  wings  are  held  erect  and  the  legs  are  feeble. 

2.  This  character  follows  the  sex-linked  method  of  trans- 
mission and  shows  linkage  with  bar  and  white  eye, 

3.  The  point  of  principal  interest  is  that  in  mating  females 
heterozygous  for  the  factor  with  erect  males  the  homozygous 
females  which  presumably  would  be  erect  do  not  appear  and  con- 
sequently there  are  twice  as  many  males  as  females. 

4.  The  lethal  effect  and  the  character  erect  seem  to  be 
manifestations  of  the  same  factor  since  it  has  not  been  possible 
to  separate  them  in  the  several  thousand  flies  which  have  been 
raised. 

5.  Erect  males  are  so  ill  adapted  to  life  under  natural 
conditions  that  they  may  be  regarded  as  lethal -bearing  males 
which  under  laboratory  conditions  are  permitted  to  live  long 
enough  to  propagate.  Females  receiving  two  of  these  lethal  fact- 
ors die  earlier  in  the  life  cycle.  This  indicates  a greater 
effect  of  the  two  erect  factors  of  the  female  as  compared  with 


19 


a single  erect  factor  of  the  male* 

6.  The  single  erect  factor  of  a female  supposedly  is  with- 
out effect  hut  it  is  of  interest  to  note  that  a dominant  acces- 
sory factor  has  appeared  which  makes  the  erect  factor  dominant 
so  that  these  females  have  erect  wings  and  occasionally  feeble 
legs  like  the  males. 

7.  There  is  some  evidence  which  seems  to  indicate  that  two 
doses  of  this  accessory  factor  also  make  the  lethal  manifestation 
of  the  erect  factor  dominant.  This  would  explain  the  apparent 

scarcity  of  heterozygous  females  and  erect  males  in  tables  1 and 


. 

. 


. 


, 


. 


20 


BIBLIOGRAPHY 

Bridges,  C.B.  1920  White  ocelli  - An  example  of  a slight 
mutant  character  with  normal  viability.  Biol.  Eull. 
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Little,  C.C.  1917  The  relation  of  yellow  coat  color  and 
black-eyed  white  spotting  of  mice  in  inheritance. 
Genetics  vol.  2,  pp.  433-444. 

Mohr,  O.L.  1919  Character  changes  caused  by  mutation  of  an 

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Mohr,  O.L.  and  Sturtevant,  A.H.  1919  A semi -lethal  in  Dros- 
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t 


21 


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Vol.  30,  pp.  293-324. 


♦ 


t 


. 


t 


% 


« 


Figure  1.  ./In  erect  male*  ( Sketched  under 
camera  lucida  while  lightly  etherized.) 


